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  <title>promise的几个关键问题4</title>
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  <script>
    /* 
    5.promise如何串连多个操作任务?
      (1)promise的then()返回一个新的promise, 可以开成then()的链式调用
      (2)通过then的链式调用串连多个同步/异步任务
    */
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("执行任务1(异步)")
        resolve(1)
      }, 1000);
    }).then(
      value => {
        console.log('任务1的结果: ', value)
        console.log('执行任务2(同步)')
        return 2
      }
    ).then(
      value => {
        console.log('任务2的结果:', value)
        
        return new Promise((resolve, reject) => {
          // 启动任务3(异步)
          setTimeout(() => {
            console.log('执行任务3(异步))')
            resolve(3)
          }, 1000);
        })
      }
    ).then(
      value => {
        console.log('任务3的结果: ', value)
      }
    )

  </script>
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